Induction melting systems have gained in popularity in the production of metal cast parts, because they provide an efficient and clean way to heat and melt metals. In an induction furnace, a metal charge is placed inside, or immediately adjacent to, an induction coil. Electric power is supplied to the induction coil, and heats the metal by means of the electromagnetic field produced by the coil. Modern induction melting systems include an induction furnace supplied with power through a solid state power converter. The solid state power converter converts three-phase standard-frequency (50 or 60 Hz) power, from a public power utility's distribution line or the like, into single-phase variable-frequency current applied to the coil. The converter adjusts the variable frequency to match the inductive impedance of the coil with the capacitive impedance of the power supply to deliver optimum power to the metal charge inside the furnace.
Advances in power semiconductor devices have made it possible to build larger static solid state converters capable of providing megawatt level power. This facilitates production rates of tens to hundreds of tons of molten metal per hour.
The increase in size ofinduction melting systems poses two major requirements--continuous uninterrupted supply of molten metal "on-spec" from the melt shop to the cast shop and high reliability of the melting system. U.S. Pat. No. 5,272,719 describes a dual output converter that allows a single transformer/rectifier unit to deliver power to two furnaces simultaneously and allows the power to be shifted smoothly between the two furnaces.
This concept may be further expanded to allow the use of a plurality of inverter units, each supplying a separate furnace, with one rectifier unit. A large melt shop may require production of 100 tons of molten metal each hour consuming about 50 megawatts. To assure a constant and steady supply of metal, the system may consist of a transformer/rectifier unit capable of converting 50 megawatts from AC line to DC and three 24-megawatt inverters each connected to a 35 ton furnace. With such an arrangement, full power melting can be conducted in two furnaces simultaneously, consuming 48 megawatts, while 2 megawatts can be applied to maintain the temperature in the third furnace which is in the process of dispensing hot molten metal.